1. Field of the Invention
This invention relates to glass fibers for optical transmission (to be hereunder referred to as an optical fiber).
2. Description of the Prior Art
Various methods have been proposed for coating optical fibers. Most of those are already implemented or expected to be implemented using a basic concept typified by the method described in U.S. Pat. No. 3,980,390. According to this method, a meltspun optical fiber, prior to its contact with another solid object, is coated with a resin composition which is further coated with a thermoplastic resin composition through melt extrusion. The first coat of the resin composition (hereunder referred to as a primary coat) formed on the fiber immediately after its spinning serves to retain the virgin strength of the glass, and the thermoplastic resin coat (hereunder referred to as a secondary coat) formed on the primary coat through extrusion enhances the mechanical strength of the fiber and protects it from subsequent mechanical stress, moisture in the air or ultraviolet radiation. The two-layer fiber system produced by this method is illustrated in FIG. 1 wherein 1 is a glass fiber, 2 is a primary coat and 4 is a secondary coat (the fiber of this type will hereunder be referred to as fiber A).
However, fiber A is known to sustain a fluctuation in transmission characteristics due to "microbending" as reported by D. Gloge et al., "Optical-fiber packaging and its influence on fiber straightness and loss", BSTJ, 54, 1975, pp. 245-262, and several methods have been proposed to improve the two-layer structure of FIG. 1. One typical example is shown in FIG. 2 wherein a buffer layer 3 made of a material having a small Young's modulus (such as a silicone resin, a rubber-like material such as butadiene resin, foamed plastic, and ethylene-vinyl acetate copolymer) capable of absorbing external stress is provided between the primary and secondary coats (a fiber of this type will hereunder be referred to as fiber B). Another example is illustrated in FIG. 3 wherein the inside diameter of the secondary coat is made larger than the outside diameter of the primary coat to provide an open space between the two coats (a fiber of this third type will hereunder be referred to as fiber C).
These improvements are characterized by mechanical isolation of the primary and secondary coats so that no external stress or stress due to the high thermal expansion coefficient of the thermoplastic resin of which the secondary coat is made is transmitted to the fiber. As already reported by research and confirmed experimentally by the inventors, unlike fiber A of the structure shown in FIG. 1, fiber B and fiber C as illustrated in FIGS. 2 and 3 sustain only a small increase in transmission loss upon application of external pressure or under cold conditions. Because of the open space provided between the primary and secondary coats, fiber C is highly resistant to microbending which occurs due to external force or thermal stress, but if the secondary coat shrinks longitudinally, the fiber will become serpentine. To make the cycle in which the fiber becomes serpentine greater than the cycle of microbending, the inside diameter of the secondary coat is increased and this results in a fiber whose outside diameter is about 2 mm, thus sacrificing the greatest advantage of optical transmission (greater transmission capacity for a unit cross-sectional area). Consequently, the inventors' efforts have been directed to improvement of fiber B.
Several thermoplastic resins that can be melt-extruded to form the secondary coats have been proposed to date, and the actual use of polyamide, high density polyethylene, polycarbonate and polyester has been reported in many prior art references because they are easy to form by extrusion, have high weatherability, and have relatively high mechanical strength. The most popular resin is the polyamide which has a relatively small thermal expansion coefficient for plastics, has been used for many years as a material in coating electric wires, and permits the use of an ordinary adhesive for splicing with a connector. The inventors have made various studies of fiber B using polyamide as the material for the secondary coat and successfully produced a fiber which is substantially free from fluctuations in transmission loss due to "microbending", namely fluctuation that occurs due to extrusion of the secondary coat and a fluctuation due to the effect of external stresses developing during the assembling and sheathing steps or cable-laying.
In addition to the resistance to microbending, the optical fiber must have stable transmission characteristics at temperatures from -40.degree. C. to 60.degree. C., and if it is to be used as a submarine cable, which is one possible future application of optical fibers, its characteristics must also include stability under high water pressure. According to the studies of the present inventors, fibers A, B and C when coated with polyamide suffered increased transmission loss of varying degrees when they were exposed to a temperature lower than -40.degree. C. Fiber A experienced the greatest loss, fiber C suffered the second greatest loss, and fiber B was the least susceptible to low temperatures. A plausible reason for the increased transmission loss in fibers A and B at low temperatures is as follows: polyamides (and other thermoplastic resins) have an expansion coefficient more than 10 times greater than glass and, therefore, shrink under cold temperature and distort to cause a slight bend in the fiber which then results in an increase in transmission loss. Because of the buffer layer fiber B is far more stable than fiber A under cold temperatures, but a further improvement is necessary for fiber B to keep its characteristics stable at a temperature lower than -40.degree. C.
When optical fibers are used in a submarine cable, if any accident should cause the cable sheath to break and sea water enters the cable, the optical fiber will be placed under a pressure proportional to the depth of water where the cable is laid. Therefore, the fiber is required to keep its stable characteristics even at a pressure higher than 100 kg/cm.sup.2.
According to the studies of the present inventors, the transmission loss occurring in fibers A, B and C will increase at a water pressure higher than 100 kg/cm.sup.2. The increase in transmission loss is proportional to the pressure, but the loss will diminish as the level of pressure decreases. Water pressure acts on the optical fiber equally in all directions, but the slightest heterogeneity in the nature of the coating will probably cause microbending. It was therefore concluded that the increased transmission loss at -40.degree. C. and at a water pressure higher than 100 kg/cm.sup.2 may be attributable to the same mechanism (i.e., development of microbending as a result of the clamping effect of the secondary coat on the interior). If so, the clamping-induced microbending can be prevented by either (1) enhancing to a greater extent the ability of the buffer layer of FIG. 2 to absorb external stress, in other words, using a material of even smaller Young's modulus or increasing the thickness of the buffer layer or by (2) reducing the clamping effect of the secondary coat. If the Young's modulus of the material for the buffer layer is decreased excessively, a buffer layer being formed by extrusion may come off the primary coat due to mechanical contact. In addition, the buffer layer is formed by baking a film of thermosetting resin and, therefore, a thicker buffer layer reduces the production speed of optical fibers or requires large production equipment.